The neurophysiology of perceptual and cognitive processing can be investigated in humans by the non-invasive recording of event-related potentials (ERPs) from the scalp. The ERPs are voltage deflections in the EEG that represent neuronal activity ERP methodology to analyze the brain systems and processing mechanisms that underly several varieties of visual perception and attention. The general approach is to record specific ERP components that index different levels of visual information processing and attentional selection. Five experiments are proposed, each of which provides converging evidence for the behavioral/psychophysical studies proposed in other sections of the Program Project. Experiment 1 will investigate the role of cortical-subcortical interactions in perceptual processing and visual-spatial attention; the design will involve combined behavioral/ERP studies of interhemispheric interaction in visual search and target detection in commissurotomy (split-brain) patients. Experiment 2 will further examine interhemispheric interactions during focussed and divided attention and will attempt to localize the neural substrates of attentional control processes that are lateralized to one or the other hemisphere. ERP component localization will be carried out by means of 32- channel topographical mapping of scalp potentials with current source density analyses. Experiment 3 will use ERP measures of visual spatial attention to test the hypothesis that disengagement of attention is a critical factor in the production of very short latency (express) saccades; the experimental paradigm will be identical to that used in Project 4. Experiments 4 and 5 will examine mechanisms of interaction between global and local levels of processing in visual pattern perception; these ERP experiments will complement the psychophysical studies of spatial frequency channels in vision to be carried out in Project 2.